1. Field of the invention
This invention relates to a method of preparing particles of a vitreous enamel for use in forming a vitreous enamel body by fusing the particles together to form the body. The invention includes a process of manufacturing such a vitreous enamel body.
The expression "vitreous" is used herein to denote materials which include a glassy phase. Vitreous materials thus comprehend partially vitrified materials which also include one or more crystalline phases as well as wholly vitrified materials.
As is well known, enamel bodies such as coatings are applied to a wide variety of articles for decorative purposes, to facilitate cleaning, to protect the article, or for some other reason or combination of reasons. Such articles may be of a ceramic nature, for example pottery, earthenware, vitreous china, stoneware or porcelain, or they may be of metal, for example cast iron. Examples of particular products include bricks, tiles, sanitary ware, bath-tubs and domestic cookers.
2. Description of the Related Art
In the classical enamelling process as usually applied industrially, a base enamel is chosen for its composition in order that it may confer appropriate properties on the enamel glaze formed, taking into account the nature of the article being glazed. Such a base enamel may be in the form of a single frit, or it may be constituted by two or more base frits. A wide variety of such base frits is commercially available. Such base frits are often compounded to result in a transparent colourless glaze, and, unless a transparent colourless enamel glaze is required, it is the normal commercial practice to mix the base frit with a nonvitreous colouring agent and/or opacifying agent such as talc, kaolin or zircon. Colouring and opacifying agents are also widely available commercially. In commercial practice, a premixed enamel composition may be bought from an enamel supplier for application to the articles to be glazed, or a base frit and additives, such as colouring agents for addition to the frit may be purchased separately and mixed as desired. The various ingredients, which are available in the form of crushed particles, are mixed together in appropriate proportions and are formed into rather friable agglomerated granules. In order to apply the enamel, the agglomerated granules of mixed frit and additives are emulsified to form a slip or barbotine which is then applied to the article to be glazed, whether as a substantially uniform coating or in some pattern, and the article is placed in a kiln where the enamel is fused together and to the article.
Such a process suffers from a number of disadvantages. It is very difficult to ensure constancy of application of enamel of uniform composition during a lengthy production run. The slips may not be stable due to decomposition of the emulsifying agents used. The slips are usually applied to the articles by spraying, and this is messy and wasteful, and requires frequent cleaning of the working area. Furthermore, the waste material may well contain boron or other materials which cannot be discharged to waste without taking special precautions. Also, the firing of the enamel requires a great deal of energy.
The known particles may also be applied by a dry technique to a substrate but this technique also suffers from disadvantages. It is difficult to distribute and fix dry ground enamels onto a substrate. Moreover, they absorb humidity forming lumps which make their uniform distribution even more difficult.